The electoral rise of Islamist parties following the Arab uprisings has led to different political outcomes in different countries, generating a significant amount of critical engagement with the concept of post-Islamism that emerged two decades ago. This article examines the notion of post-Islamism and uses one important aspect of it to explain the continued relevance of thinking about Islamist politics through ideology. Rather than being a failure for its acceptance of democratic mechanisms—and the apparent renunciation of the creation of an Islamic state—Islamism can be thought of as an evolving ideology within which the legitimation for the Islamic state is found still in religious sources and interpretations. In order to examine this aspect of post-Islamism, the article analyses in detail the case of the Tunisian Islamist party al-Nahda.